1. Field of the Invention
The present invention relates to screw conveyors and, in particular, to methods and apparatus for manufacturing the screw flight sections of screw conveyors.
2. Description of the Prior Art
The screw flight section of a screw conveyor is a helical ribbon that is attached to a shaft so that material will be conveyed along the length of the shaft when the assembly is rotated about its longitudinal axis.
In the prior art screw flights have usually been fabricated by one of three methods. None of these previous methods has been wholly satisfactory.
One method, a hot-rolling method, produces continuous flights by treating flat bar stock in a rolling machine. Its use, however, has been limited because it has the disadvantage of causing thinning of the more wearprone outer portion of the flight.
A second method, a stretch method, produces the flights as a weldment of partially formed sections made from blanks of split-ring configuration. After welding, the assembled flight is stretched to the required length. This method eliminates the thinning problem, but has the disadvantage of producing variations in pitch of the flight's screw form. This pitch variation reflects difference in helix angles that result from large variations in the thickness of the blank material.
The third method, the stamping method, attempts to minimize pitch variations by forming these split-ring blanks between dies. This method provides better control of helix angles and has been the most popular means of producing the flights. One disadvantage of this method is its high tooling costs. A separate set of dies is required for each combination of diameter, pitch, and thickness and grade of material used. A second disadvantage is the need for frequent and skillful adjustment of dies and press equipment to compensate for the unavoidable differences in material thickness and springback properties.